The invention relates to an apparatus for the automatic determination of a continuous bulk material throughput by means of a continuous balance, comprising a weighing vessel, an adjustable closure member for adjusting the outlet cross-section of the weighing vessel and electronic weight difference measuring means. The invention also relates to a process for the automatic determination of a bulk material throughput in such an apparatus.
Apparatus of the foregoing type have been known. For example, CH-A-557 060 discloses a process and an apparatus for the automatic determination of a bulk material flow. The basic idea used in this prior art is essentially based on the deflecting action of a butt or deflecting plate (baffle plate). Thus, if a falling bulk material flow is deflected by a butt or baffle plate placed in its line of fall, a momentum acts on said plate. The momentum force is dependent on the height of fall of the in each case instantaneous bulk material throughput and is also influenced by the position, shape and friction characteristics of the plate, but particularly by the bulk material behaviour. Other influencing parameters are the product granulation, flow behaviour, air and product moisture, temperature, etc. This list in itself shows that for the continuous determination of the mass flow of the flowing product very complex relationships are involved, despite the constructional simplicity of the actual baffle plate used. The problems which occur could only be solved in practice by accepting as invariable quantities certain framework conditions. Thus, the first restriction was that the deflecting plate system can only be used for really freely flowing material, such as grain. The product flow must be guided by geometrically constant conditions. It is theoretically conceivable to use the momentum established by the measuring or test plate for determining a bulk material throughput.
However, it has been found that the more practicable and appropriate way is to use the horizontal component of the momentum only, as is shown e.g. by DE-C-2 609 167 as a further development of the aforementioned CH-A-557 060.
The vertical component of the momentum, i.e. the "weight component", is eliminated in this type of test plate system by corresponding articulated supports for measured value determination. The horizontal deflection or the corresponding horizontal component from the interplay of the falling bulk material flow and the deflecting plate can in this way be used as a measured or test value for the automatic determination of the throughput of the bulk material flow.
Under laboratory conditions, this method makes it possible to obtain a measurement accuracy of .+-.0.5% and in very many cases .+-.1%. However, under more difficult conditions larger deviations individually appear. However, it is conventional practice to regularly guarantee bulk material balance accuracies of .+-.0.1%.
However, in all cases where balance accuracy is required, balances are still used although they are unable to measure a continuous bulk material flow and must instead interrupt the product flow for weighing purposes. In addition, balances are not only more expensive than systems with deflecting plates, but frequently need compensating elements, so that after the container balances a continuous product flow is obtained again. Conveyor scales or weighers with a high accuracy are even more expensive than container balances and often do not achieve the accuracy of the latter.
In the case of the presently observed intense automation efforts, particularly in mills, increasing importance is being attached to the following two points:
1. The product flow must have a maximum continuity with minimum fluctuation;
2. It must be possible to determine the throughput of the continuous product flow approximately with balance accuracy.
Of late, so-called differential balances have often been used, such as is e.g. shown by FR-A-2 456 344. The differential balance comprises a weighing container with a controlled product discharge, as well as a dosing means controlled by said container. The weighing container measures the material removed from the product discharge point or the running weight loss in the container. The measured results are in themselves very accurate, but a disadvantage of the known differential balances is that it is necessary to adhere to a regular sequence between filling and emptying. Although a discharge can take place during filling, the disadvantage then arises that during the filling phase the measured values of the balance are disturbed and rendered unusable by the inflowing product flow. For this purpose the aforementioned French specification proposes controlling the discharge member on the basis of a volumetric throughput during the filling phase. However, it is necessary to accept the corresponding errors.
In the case of fuel feeding of e.g. furnaces, less importance is attached to the accuracy than e.g. when processing foods in mills, particularly when determining the throughput of a grain flow, e.g. for the simultaneous precise dosing in of the missing water quantity, or for supplying and partly controlling processing machines.
A further problem in mills is the mixing of different grain types for which, hitherto, particularly high demands have not been made. However, this has changed through using computers for controlling the complete product flow, because the processed product quantity must be accurately determined, because otherwise over long periods there can be major differences between the actual grain quantities present in the individual storage compartments and the summated falsed data due to imprecise measurement. Corresponding to the number of storage compartments or product runs, a large number of throughput measuring points are required, so that the use of expensive weighing systems is not economically viable due to the large numbers necessary.